In International Publication No. 2011/024506, there is disclosed an antenna device in which antenna matching that has deviated due to the influence of the surroundings, such as there being a human body nearby, is corrected each time such deviation occurs.
FIGS. 13(A) and 13(B) show a configuration diagram of the antenna device described in International Publication No. 2011/024506. FIG. 13(A) is a configuration diagram of the entirety of the antenna device and FIG. 13(B) illustrates the basic form of a capacitance-voltage conversion circuit.
In FIG. 13(A), a variable matching circuit 30 is provided along a wireless communication signal path, which is a transmission path between an antenna element 21 and a feeder circuit 40. A capacitance detection circuit 60 forms a capacitance-voltage conversion circuit (C-v conversion circuit) that converts a change in stray capacitance generated by the nearness of a human body into a change in voltage and then outputs the change in voltage. A feedback control circuit 70 supplies a control signal to the variable matching circuit 30 on the basis of the voltage output from the capacitance detection circuit 60.
In FIG. 13(B), a local oscillator OSC is connected in series with a detection target capacitance Cs. A reference potential Vref1 is applied to a non-inverting input terminal of an operational amplifier OP. Consequently, a potential at a connection point P5 between the detection target capacitance Cs and a feedback capacitance Cf (input voltage of operational amplifier OP) is a potential corresponding to the detection target capacitance. Compared with the frequency band of a wireless communication signal, an oscillation frequency of the local oscillator OSC is a low frequency that is almost the same as that of a direct current.